An optical disc apparatus for the next generation is required to operate with a large capacity of information and the high information bit rate recording. For this purpose, an objective lens for high information bit rate recording is used for an optical head, and is required to be controlled with a high speed and the high precision. Therefore, an objective lens actuator to be moved in focus and track directions must be small in size and light in weight, and must generate a driving force to overcome the disc acceleration.
As an example of conventional apparatus, we can quote the objective lens actuator which appeared on Japanese Patent Kokai No. 3-156730. The outline of the aforementioned apparatus is as follows. A focus drive coil is a rectangular-shaped one and wound around on outer periphery of a lens holder. A pair of parallel tracking drive coils are fixed to opposite sides of a focus drive coil, such that coils surface of the formers and the latter are perpendicular to each other.
The lens holder and aforementioned coils are placed in DC magnetic field caused by a pair of parallel plate magnets which are fixed to the actuator base and magnetized in opposite directions.
In operation, the current in the focus drive coil is proportional to the focus servo error signal corresponding to the focus drive direction, and the electro-magnetic force which is generated by this coils is proportional to the product of the coil current and the external magnetic flux density, i.e. the one caused by aforementioned plate magnets, and the direction of the electro-magnetic force is perpendicular to the coil surface. Therefore, the focus drive coil moves up and down corresponding to the polarity of the coil current, until the electro-magnetic force balances with restoring forces of four wires which support the lens holder. In the case of the tracking drive coils, all situations are quite similar to those of the focus drive one, however, the coils moves to the right and the left according to the servo error signal corresponding to the tracking drive direction. Thus, focus and tracking adjustments are carried out in accordance with servo error signals supplied from an optical unit, which contains various optical hardwares and produces focus and tracking error signals, and information signals.
On the aforementioned objective lens actuator, however, following disadvantages have been pointed out. The structure for four-wire-suspension which was designed to move objectives lens in two orthogonal axes causes rotational motions in focus and tracking directions. Moreover, for objective-lens actuator, it is difficult to increase the drive sensitivity of tracking drive coils, because dimensions of these coils and external DC magnetic fields acting to these coils cannot be so large, by the reasons of spacial limitations and structures of magnetic circuits respectively. Furthermore, a mechanism for adjusting initial position and attitude of the lens holder in both tracking drive and tangential (perpendicular to tracking) direction becomes complicated, when larger range of adjustment is desired.